Composite golf club shafts typically have hollow tubular bodies that taper longitudinally from larger, so-called “butt” or “grip” ends toward smaller, so-called “tip” ends upon which golf club heads are mounted in the completed golf clubs. Such shafts typically are generally circular in transverse cross-sectional shape, both at the outside and inside surfaces of the shaft, having walls that are of selected thicknesses and compositions to provide the strength, flexibility and weight desired for a particular golf club.
The design and manufacture of composite golf club shafts are highly developed arts, providing a wide variety of different shafts with characteristics that are intended to suit the abilities and personal preferences of a wide variety of golfers. Typically, composite shafts are designed to be concentric about their longitudinal axes while varying substantially in outside diameter, from the larger grip end to the smaller tip end. The concentricity of the inside and outside surfaces is designed to be very precise, to produce the desired wall thickness and flexing characteristics, and remains stable when at rest, that is, when not loaded and stressed by outside forces.
During the swing, however, the forces acting on the shaft as the club is swung through the golf stroke are great enough to deform the shaft, longitudinally in flexing along the length of the shaft and torsionally in twisting of the shaft, and also transversely, causing the cross-sectional shape of the shaft to deform and become oval or elongated. This deformation is resisted by the wall strength of the shaft, referred to as “hoop strength”, but occurs in different degrees and directions, first in the so-called “swing plane (or planes)” of the golfer's swing and secondarily in the so-called “droop plane” that is generally perpendicular to the swing plane. The amounts of these deformations are functions of the forces applied throughout the swing and ball impact, and the physical properties of the shaft resisting these forces.
In the industry, various approaches are available to provide the desired properties in the shaft for improved performance, including increasing the wall thickness and the amounts of different composite materials in the wall, and varying the angles of the fibers in the composite materials relative to the longitudinal axis of the shaft.
Increased use of so-called “angle fibers” provides increased transverse wall strength. All such changes affect other performance characteristics of the shaft, including weight and longitudinal and torsional flexibility. In general, the technology of design and manufacture of golf club shafts, including the selection, placement and use of different types and angles of fibers, are well known in the industry to those skilled in the art, and this information therefore is included only as general background for the present invention. The present invention is directed to a novel improvement in golf club shafts that contributes significantly to the hoop strength of a golf club shaft to improve its performance characteristics without adversely affecting other performance characteristics of the shaft.